fix(SlideDrag) clears drag when list is scrolled or new drag starts

This also fixes the broken $ionicListDelegate.clearOptionButtons() method.

Makes lists feel more natural inline with iOS functionality and fixes issues
with using ion-option-buttons with collection repeat
Merge pull request #2558 from mallzee/slide-drag-clear

fix(SlideDrag): clears drag when list is scrolled or new drag starts